Postjacking 

When someone comments on a Facebook post such as a wallpost, status update, or anything else in an attempt to bring in another conversation and divert attention away from the post and toward themselves.

Jason posted on Marie's Wall:
"Hey...how have you been?"
Jack commented on Jason's wall post:
"JASON! Why didn't you text me back?"

And Jason and Jack have a conversation on the wall post that was meant to be for Marie, and thus Jack postjacked the conversation. Postjacking can also occur in status updates, but it most common on wall posts.

Postjacking by Johanson Johnson December 27, 2009

postjack 

Antonym of prejack, postjack is the capacity, (exclusive to men), to think clearly, make responsible purchase decisions, refuse to comply with the suggestions of hot women, do math, and pass strip clubs without going inside. Typically a momentary, fleeting state that goes away a few minutes "postjack".

Postjack it’s pretty clear to me how much I hate that fucking couch she made me buy, and how – frankly – I’d really like to strangle her goddamn cat… but the feeling usually subsides after a couple minutes, confusion sets in, and I have to go look for my calculator and a dictionary… wait, what was I talking about? I need to go look at redtube.

postjack by Beef Ranger October 22, 2008

postjacking 

"Postjacking" is when a couple or a few people take over a comment string, usually on Facebook, and subsequently take things in a whole different direction. This is not inherently a negative thing, as it can often be amusing to witness.

Sorry for postjacking your string, Dude! We'll let things get back on topic now.

postjacking by Plasmo March 6, 2015